1. Field of the Invention
This invention relates to a wafer stage device, control system, and method for stabilizing a wafer stage and a wafer table in a photolithography process to manufacture semiconductor wafers. More particularly, this invention relates to a device, control system, and method for stabilizing the wafer stage and the wafer table to prevent pitching and rolling motions of the wafer table when the wafer stage moves.
2. Description of the Related Art
In manufacturing integrated circuits using photolithography, light is transmitted through non-opaque portions of a pattern on a reticle, or photomask, through a projection exposure apparatus, and onto a wafer of specially-coated silicon or other semiconductor material. The uncovered portions of the coating, that are exposed to light, are cured. The uncured coating is then removed by an acid bath. Then, the layer of uncovered silicon is altered to produce one layer of the multi-layered integrated circuit. Conventional systems use visible and ultraviolet light for this process. Recently, however, visible and ultraviolet light have been replaced with electron, x-ray, and laser beams, which permit smaller and more intricate patterns.
As the miniaturization of a circuit pattern progresses, the focus depth of the projection exposure apparatus becomes very small, making it difficult to align accurately the overlay of circuit patterns of the multi-layered integrated circuit. As a result, a primary consideration for an overall design of the photolithography system includes building components of the system that achieve precision by maintaining small tolerances. Any vibration, distortion, or misalignment caused by internal, external or environmental disturbances must be kept at minimum. When these disturbances affect an individual part, the focusing properties of the photolithography system are collectively altered.
It has been proposed to provide a conventional wafer stage device 100 as shown in FIGS. 1 and 2. In the device 100, a wafer table 104 supports a semiconductor wafer 68. A wafer stage 66 positions wafer 68 as wafer stage 66 is accelerated by a wafer stage force Fs generated in response to a wafer manufacturing control system (not shown). A set of flexures 106 connects wafer table 104 to wafer stage 66. According to the conventional wafer stage device 100, flexures 106 are positioned on a plane A that is substantially in alignment with the center of gravity (c.g.) of wafer table 104. When wafer stage force Fs acts on wafer stage 66, flexures 106 apply a wafer table force Ft in plane A on wafer table 104.
The conventional wafer stage device 100 has several disadvantages, including a leveling shift problem. A leveling shift problem occurs because flexures 106 and an upper surface of wafer 68 are separated by a distance D1 along the z axis. In some embodiments, distance D, may be as large as 30-40 mm. This separation leads to a horizontal shift in the position of wafer 68 when wafer table 104 is tilted to maintain proper focus. The horizontal shift along the x- and y- directions as measured by an interferometer reflected from mirror 110 can be expressed as follows:
xe2x80x83Xshift=D1xc2x7xcex1y
Yshift=D1xc2x7xcex1x
whereby, xcex1x, and xcex1y are the tilt angles of wafer table 104 with respect to the x and y axes, respectively.
In light of the foregoing, there is a need for a wafer stage device, control system, and method for stabilizing the wafer stage and the wafer table so that the pattern on the reticle can be accurately projected onto the semiconductor wafer during exposure in the wafer manufacturing process. Particularly, there is a need for a wafer stage device that can overcome the leveling shift problem and minimize pitching and rolling errors.
The advantages and purposes of the invention will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The advantages and purposes of the invention will be realized and attained by the elements and combinations particularly pointed out in the appended claims.
To attain the advantages and consistent with the principles of the invention, as embodied and broadly described herein, a first aspect of the invention is a wafer stage control system, comprising a first feedforward control system to determine a stabilizing force to be exerted onto a wafer table to counteract a rotational motion of the wafer table caused by an acceleration of a wafer stage, and a second feedforward control system to determine a correction force to-be exerted on the wafer stage to correct a leveling shift caused by a rotation of the wafer table.
A second aspect of the present invention is a method for operating a wafer stage and wafer table control system. The method comprises the steps of determining a stabilizing force to be exerted on a wafer table to counteract a rotational motion of the wafer table caused by an acceleration of a wafer stage, and determining a force to be exerted on the wafer stage to counteract a leveling shift of the wafer table due to a rotation of the wafer table.
A third aspect of the present invention is a method for operating a wafer stage and wafer table control system. The method comprises the steps of creating a wafer stage loop to correct a wafer stage position error, a wafer stage acceleration reference signal being an input to the wafer stage loop, and creating a wafer table loop to correct a wafer table position error. The method also comprises the steps of inputting the wafer stage acceleration reference signal to the wafer table loop to determine a stabilizing force to be exerted onto the wafer table to counteract a rotational motion of the wafer table caused by an acceleration of the wafer stage, and determining a force to be exerted on the wafer stage to correct a leveling shift of the wafer table due to a rotation of the wafer table.
A fourth aspect of the present invention is a method for stabilizing a wafer table in a wafer manufacturing apparatus. The wafer table supports a wafer, and the wafer stage is being subject to acceleration in response to a wafer manufacturing control system to position the wafer table. The method comprises the steps of generating a wafer stage force to be exerted onto a wafer stage to correct a position error of the wafer stage, and generating a wafer table force to be exerted onto a wafer table to correct a position error of the wafer table. The method also comprises the steps of generating a stabilizing force to be exerted onto the wafer table to counteract a rotational motion of the wafer table caused by acceleration of the wafer stage, and generating a correction force to be exerted onto the wafer stage to compensate leveling shift of the wafer table.
A fifth aspect of the present invention is a method for making a wafer stage device to manufacture semiconductor wafers. The method comprises the steps of aligning at least one flexure in a plane with a wafer, the wafer being supported by a wafer table and positioned by a wafer stage, and restraining shifting motion of the wafer table in the plane aligned with the wafer, while allowing the wafer table to move in a direction normal to the plane. The method also comprises the step of generating a stabilizing force to stabilize the wafer table when the wafer stage is accelerated.
A sixth aspect of the present invention is a wafer stage device for manufacturing semiconductor wafers, comprising a wafer stage to position a wafer, the wafer stage being subject to acceleration in response to a wafer manufacturing control system, and a wafer table to support the wafer, the wafer table having a predetermined center of gravity. The wafer stage device also comprises a set of flexures connecting the wafer table and the wafer stage, the set of flexures being substantially aligned in a plane with the wafer, and at least one actuator to stabilize the wafer table when the wafer stage is being accelerated. The wafer stage device further comprises a first feedforward controller to control a stabilizing output of the at least one actuator.
A seventh aspect of the present invention is a wafer stage control system for stabilizing a wafer table in a wafer manufacturing process. The wafer table supports a wafer, and a wafer stage is being subject to acceleration in response to a wafer manufacturing control system to position the wafer table. The wafer stage control system comprises at least one flexure substantially aligned in a plane with the wafer and connecting the wafer table to the wafer stage, and at least one actuator mounted on the wafer stage to generate an output to stabilize the wafer table when the wafer stage is being accelerated. The wafer stage control system also comprises a first feedforward control system to control the output of the at least one actuator by determining a stabilizing force to be exerted on the wafer table to counteract a rotational motion of the wafer table caused by an acceleration of the wafer stage.
A further aspect of the present invention is a lithography system comprising an illumination system that irradiates radiant energy and a wafer stage device that carries an object, such as a wafer, disposed on a path of the radiant energy. The lithography system may also comprise a wafer stage control system connected to the wafer stage device.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention as claimed. Additional advantages will be set forth in the description which follows, and in part will be understood from the description, or may be learned by practice of the invention. The advantages and purposes may be obtained by means of the combinations set forth in the attached claims.